1. Field of the Invention
The present invention is directed to a motor drive circuit employed for driving a motor in accordance with a servo output obtained by a servo IC.
2. Description of the Prior Art
A conventional motor servo circuit incorporates a drive IC, serving as a control unit together with a servo IC, for supplying a drive output to a motor. The drive IC in turn incorporates a motor drive circuit for supplying a drive output as well as including an output circuit for permitting a flow of a drive current to the motor.
Turning attention to FIG. 7, there is depicted a prior art motor drive circuit, in which a servo signal V.sub.IN is applied from the servo IC to an input terminal 2. The servo signal V.sub.IN, which is generated by the servo IC, consists of an FG signal showing a motor revolution and a PG signal indicating a rotary phase of the motor. The servo signal V.sub.IN represents a torque command signal for the motor to be driven. Upon an application of the servo signal V.sub.IN, a current I.sub.1 corresponding to the servo signal V.sub.IN flows via a resistance 6 into a base of a transistor 4; and a current I.sub.3 then flows into a transistor 4 and a resistance 8 as well, corresponding to the current I.sub.1. The current I.sub.3 subsequently runs into an input-side transistor 101 of transistors 101 and 102 of a current mirror circuit 10, while the transistor 102 receives an inflow of a current I.sub.2 in accordance with an emitter area ratio of the transistors 101 and 102. An output circuit 12 is supplied with the current I.sub.2 via the transistor 102, and at the same moment a power supply voltage Vcc is impressed thereon from a power supply terminal 13.
The output circuit 12 is provided with a current detecting terminal 14 for detecting an output current I.sub.OUT, and a resistance 16 for detection is connected between the current detecting terminal 14 and an earth. Based on this configuration, the current I.sub.3 passing through a transistor 4 then flows via a resistance 8 to the current detecting terminal 14. Simultaneously, the current detecting terminal 14 admits a current I.sub.10 coming from the output circuit 12. The output current I.sub.OUT obtained by synthesizing the currents I.sub.3 and I.sub.10 runs into a resistance 16, whereby a voltage V.sub.ATC corresponding to the output I.sub.OUT is generated at the current detecting terminal 14.
Connected to the output circuit 12 are Y-connected filed coils 18a, 18b and 18c of a motor 18. Applied to the output circuit 12 are control signals from a revolution control unit 22 on the basis of drive angle control signals Va, Vb and Vc applied to revolution control terminals 20a, 20b and 20c. Hence, the field coils 18a to 18c of the motor 18 are supplied with drive currents based on the control signals, and rotary outputs corresponding to the drive angle control signals Va to Vc are obtained in the motor 18. The rotary outputs are in turn controlled in accordance with the servo signals V.sub.IN.
The thus constructed motor drive circuit exhibits characteristics illustrated in FIG. 8. Set in the servo signal V.sub.IN is a threshold voltage V.sub.1 determined by a base-emitter voltage V.sub.BE of the transistor 4. The transistor 4 is brought into a non-conductive state in such a region d1 that a level of the servo signal V.sub.IN does not exceed the threshold voltage V.sub.1.
In a transition region d2 wherein the output current I.sub.OUT fluctuates in such a manner that the level of the servo signal V.sub.IN increases over the threshold voltage V.sub.1 and reaches a voltage V.sub.2, the output I.sub.OUT is given by: ##EQU1## where R.sub.1 and R.sub.2 are the resistance values of the resistances 6 and 8, and R.sub.FN is the resistance value of the resistance 16.
When an input impedance of the motor drive circuit is high, and if I.sub.3. R.sub.2 &lt;&lt;V.sub.ATC, the formula (1) is expressed such as: ##EQU2##
Therefore, it follows that the output current I.sub.OUT is controlled by the servo signal V.sub.IN.
In the region d3 wherein the level of the servo signal V.sub.IN increases in excess of the transition region d2, the output current I.sub.OUT determined depending on a limit of a drive capability of the output circuit 12 keeps a constant value even when the level of the servo signal V.sub.IN rises.
Hence, the motor drive circuit constitutes a voltage input circuit in which to determine a voltage range of a bias operating point of the servo signal V.sub.IN. An operating point of the output current I.sub.OUT is therefore univalently determined. The base-emitter voltage of the transistor which determines the threshold voltage V.sub.1 of the servo signal V.sub.IN has a typical temperature characteristic of -2 mV/.degree.C., and it follows that the threshold voltage V.sub.1 is possessed of a similar temperature characteristic. There is obviously a dependency on the temperature.